The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bone (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
A few parts of the spine include the pedicle, the laminar arch, the facet, the spinous process, the transverse process, the vertical canal and the vertebral body. The vertebral body is the cylinder-shaped weight bearing-structure of the vertebra. The lamina are flat plates on the outer wall of the vertebral canal, which is formed between the vertebral body and the lamina and occupied by the spinal cord. The pedicle connects the lamina with the vertebral body. The spinous process protrudes from the back of the vertebra such that muscles and ligaments can attach thereto. Finally, the transverse process sticks out the sides of each vertebra and are another place where muscles and ligaments can attach to the spine.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function. These spinal disorders, pathologies, and injuries limit the spine's range of motion, or threaten the critical elements of the nervous system housed within the spinal column.
One of the most common surgical interventions today is arthrodesis, or spine fusion, of one or more motion segments. Clinical success varies considerably depending upon technique and indications. Consideration also must be given to the concomitant risks and complications. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused segments. Also, the fusion device, whether artificial or biological, may migrate out of the fusion site.
A variety of devices and systems have been disclosed in the art that achieve spine fusion and immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Anterior stabilization methods include full or partial disc replacements with rigidly shaped spacers that are appropriately sized for the disc space. Posterior implants generally comprise pairs of rods that are aligned along the axis to be immobilized. The implants are attached to the spinal column by hooks coupled to the lamina or to the transverse processes or by screws inserted through the pedicles.
Other methods include flexible spinal stabilization methods that do not result in complete spinal fusion. Posterior stabilization methods may include spinal immobilization utilizing a pedicle screw and wire system. Another flexible posterior stabilization method includes the Dynesys™ system. The Dynesys™ system allows spinal segments to be placed back into position and fixed there while restricting, but not completely preventing, movement. Other systems are for the focused stabilization of particular areas of the vertebrae, such as the pedicles or facets.